This invention relates to an apparatus for reducing periodical noises transmitted into a vehicle passenger compartment from a noise source by producing control sounds for interference with the transmitted periodical noises.
For example, British Patent No. 2,149,614 published on Jun. 12, 1985, discloses a conventional noise reduction apparatus for use in airplane passenger compartments or like spaces. The conventional noise reduction apparatus is applicable to reduce noises transmitted from a single source of noises having a fundamental frequency f.sub.0 and its higher harmonics f.sub.1 to f.sub.n. The noise source is an engine or the like placed in the exterior of such a space as described above. A plurality of microphones are placed at different positions within the space for detecting the sound pressures applied thereon. In order to produce control sounds for interference with the transmitted noises, a plurality of loudspeakers are placed at different positions within the space. The loudspeakers are driven by drive signals having frequencies in opposite phase to the frequencies f.sub.0 to f.sub.n of the transmitted noises to cancel the transmitted noises. A "WIDROW LMS" algorithm developed for multiple channels is used to drive the loudspeakers. The "WIDROW LMS" algorithm is described in an article published 1975, in PROCEEDINGS OF THE IEEE, Vol. 63, page 1692, entitled "Adaptive Noise Cancellation: Principles and Applications". The "WIDROW LMS" algorithm developed for multiple channels is described in an article published 1987, in IEEE TRANS. ACOUST.,SPEECH, SIGNAL PROCESSING, VOL. ASSP-35, PP. 1423-1434 entitled "A MULTIPLE ERROR LMS ALGORITHM AND ITS APPLICATION TO THE ACTIVE CONTROL OF SOUND AND VIBRATION".
The LMS (least mean square) algorithm is one of appropriate algorithms for use in updating the filter coefficients of adaptive digital filters. For example, in a so-called Multiple Error Filtered X LMS algorithm, all of the transfer function filters modeled on the transfer functions between the loudspeakers and the microphones are set for all of the loudspeaker-microphone combinations. The filter coefficients of each of the variable filter coefficient digital filters provided for the respective loudspeakers are updated in a manner to reduce the value of a predetermined performance function calculated based upon the residual noise levels detected by the respective microphones using the reference signal, indicative of the noise generating condition of the noise source, processed for the filter.
With the conventional noise reduction apparatus, however, the reference signal indicative of the noise genera ting condition is taken in the form of a continuous signal such as a sine wave. For this reason, it is required to repeat a great number of calculations in the convolutional calculations of the reference signal and the transfer function filters and the convolutional calculations of the reference signal and the adaptive digital filters. The required calculations include multiplying the series of values obtained by sampling the continuous signal at intervals of a predetermined time and the filter coefficients of the transfer-function and adaptive-digital filters and summing the multiplied results.